Income Taxes - Schedule of Income Taxes (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes
Expected tax provision, at statutory rate	$ 11,257	$ 17,868	$ 7,231
U.S. state income taxes, net	1,036	3,298	1,372
Other, net	(350)	(328)	42
Provision for income taxes	11,943	20,838	8,645
Provision for income taxes:
U.S. federal	5,002	8,800	1,984
U.S. state	896	1,562	108
Net currently payable	5,898	10,362	2,092
Deferred income taxes, net	6,045	10,476	6,553
Provision for income taxes	11,943	20,838	8,645
Comprehensive provision for income taxes allocable to:
Net income	11,943	20,838	8,645
Retirement of treasury stock		456
Transactions with stockholders		106
Income Tax Expense	15,174	(34,211)	30,436
Defined Benefit Pension Plans [Member]
Comprehensive provision for income taxes allocable to:
Other Comprehensive Income Loss Pension plans	8,151	(49,721)	25,771
OPEB [Member]
Comprehensive provision for income taxes allocable to:
Other Comprehensive Income Loss Pension plans	$ (4,920)	$ (5,890)	$ (3,980)